Machines having varying reluctance, hereinafter called reluctance machines, may be rotating or linear machines, meaning that when a movable part rotates or moves linearly, the reluctance in the magnetic circuit of the machine will vary. In the following, this movable part is called the rotor, irrespective of the type of machine concerned. The position of this movable part is thus called the rotor position.
Normally, in order to utilize a reluctance machine optimally, the position of the movable part, i.e. the rotor position, is sensed and the machine is controlled on the basis of this sensed position. The position is often sensed with the aid of position sensors mounted on the machine, these sensors producing an output signal which denotes the absolute position. Sensors of a simpler kind which provide relative positional information, for instance increment sensors, are also used.
In recent times, methods have been proposed for controlling reluctance machines which do not include the use of physical sensors. Two such methods are described in U.S. Pat. No. 4,868,478 and S.E. 8604307-2. All of the embodiments described in these publications use different methods of sensing or measuring the inductance in a machine phase.
However, one problem which is not resolved satisfactorily by the methods described in these publications, is how the machine shall be started.
Since the inductance is indirectly a measure of the reluctance, the drive circuits described in said publications constitute examples of embodiments which can be used in practice as the basis of starting a reluctance machine. These drive circuits are not described in detail here. The invention, however, is not restricted to its use together with these drive circuits, but applies to all systems in which the reluctance or a magnitude connected thereto can be measured or sensed in the various magnetic circuits of the machine.
Machines of the kind to which the invention relates include a stationary part, here referred to as the stator, irrespective of whether the machine concerned is a rotating machine or a linear machine, and a movable part, the rotor. Both the stator and the rotor are normally made from a soft magnetic material, such is soft magnetic iron, with embossed, toothlike poles.
Flux generating windings for the different phases and poles are normally applied to the stator, which gives favourable operation with respect to mechanical construction, operational reliability, working temperature, etc., although these windings may alternatively be mounted on the rotor instead, if so desired. In the case of specific machine designs, there is used a combination of varying reluctance and permanent magnets. An example of one such design is the socalled hybrid-stepping motor.
A common feature of machines which employ varying reluctance is that these machines shall be controlled so that current will flow through the machine windings only when the reluctance for each individual winding circuit either increases with a change in rotor position, which applies to generator operation, or decreases with a change in the rotor position, which applies to motor operation. The aforesaid rotor-position-dependency derives from this.
For the sake of simplicity, the following description will be made solely with reference to rotating machines. It will be understood, however, that what is described applies also to linear machines.
The object of the invention is to provide a method for engaging and starting a machine having varying reluctance, with the intention of establishing which phase circuit or phase circuits shall be activated in order to obtain torque in a desired direction.
The aforesaid object is achieved with a method having the characteristic features set forth in claim 1. Further developments of the invention and other features thereof, and an arrangement for carrying out the method are set forth in the remaining claims.
According to the invention, the reluctance in the magnetic circuits of the machine is sensed, thereby obtaining information as to the absolute position of the rotor within a reluctance cycle. By sensing and processing this information in accordance with the inventive method, it is possible always to select the correct phase circuit, so as to obtain maximum torque in a desired direction. The method is based on the fact that the reluctance cycle can be divided into a given number of zones, where each zone has a unique relationship between the respective reluctance values of the different phases. For the purpose of determining in which of these zones the rotor is positioned, the measured values are mutually compared so to determine the order of magnitude of these values.
Although the inventive method is primarily suited for machines of the aforesaid kind which are not provided with physical rotor position sensors, the method can also be applied to machines in which rotor position sensors having limited function are used.
The invention presumes that the reluctance or a magnitude coupled thereto, e.g. the inductance, in the various magnetic circuits of the machine is measured or sensed. This measuring or sensing of said reluctance value is effected in such a way that information relating to rotor position is obtained, such that activation of the machine can be effected in a reliable manner on the basis of this information. The desired operational state of the machine is achieved in this way, e.g. the maximum torque in the desired drive direction for instance when the machine is a drive motor .